Before you can change settings of a fixture it first has to be selected:
Select a fixture:
- Click on a fixture image. (label becomes light blue)
Select more than one:
Click on other fixtures to select more than one (for example the 3 first fixtures):
To deselect all fixtures:
- Click on the background image
or
- Click on the "deselect all" icon in the toolbar 
You can change the colors of the labels for better contrast: System colors